This invention relates to compression of image-representative signals and, more particularly, to a method and apparatus for encoding and decoding image-representative signals.
Image-representative signals can be digitized, encoded, and subsequently decoded in a manner which substantially reduces the number of bits necessary to represent a decoded reconstructed image without undue or noticeable degradation in the reconstructed image. Coding methods that use transforms, for example wavelet transforms, are well known in this art.
One known approach to image compression is to identify individual objects in an image and to encode information about each object for subsequent recovery at a decoder. Techniques are known for specifying the shape and location of the object, such as by producing a bitmap which precisely defines which pixels are within the object (e.g. by using 1""s within the object and 0""s outside the object). As is also known, the object can be represented by a wavelet transform which, in conjunction with the known techniques of eliminating certain transformed information, results in a compressed encoded bit stream that can be decoded for reconstruction of the object with substantial saving of bandwidth. However, there are several problems encountered when transforming the pixels of an object with a wavelet transform. As will be described further herein, problems are encountered in handling boundaries and doing so in a way that maintains the integrity of the coding without being wasteful of bandwidth. For small objects, or small shape features of larger objects, the handling of coding at boundaries under certain conditions can have a very substantial effect on compression efficiency.
It is among the objects of the present invention to provide improved techniques and apparatus for encoding objects of arbitrary shape to achieve efficient image compression.